The invention relates to a method for the selective catalytic oxidation of carbon monoxide (CO) in the presence of a noble metal catalyst on an alumina carrier.
Fuel cells are being investigated in many places as a possible energy source for driving vehicles and for stationary generation of electricity. The use of fuel cells is still highly dependent on the availability of the fuel: hydrogen (H2). It is not to be expected that an infrastructure for hydrogen will be set up within the foreseeable future. Especially for mobile applications, it is therefore necessary to transport an available fuel, or a fuel that becomes available, and to convert this to hydrogen as the feed for the fuel cell.
A gas mixture that consists mainly of hydrogen and carbon dioxide (CO2) is then producedxe2x80x94for example via steam reforming and/or partial oxidationxe2x80x94from fuels such as methane, LPG, methanol, petrol, diesel and other hydrocarbons. Said gas mixture, which is rich in hydrogen, is then fed to the fuel cell which generates electricity by an electrochemical reaction of hydrogen with oxygen.
However, a certain amount of carbon monoxide (CO) is also always liberated during the conversion of said fuels into hydrogen. For instance, a gas mixture of, for example, 75% (V/V) H2, 24% (V/V) CO2 and 1% (V/V) CO is produced on steam reforming of methanol. A solid polymer fuel cell, the major candidate for transport applications, is extremely sensitive to CO, which even in low concentrations (0.01% (V/V)) has an adverse effect on the performance of the fuel cell. For a usable system it is therefore necessary to remove CO down to the said level and preferably down to a lower level ( less than 0.005% (V/V), 50 ppm). A technically attractive option for removing CO from H2-containing gas streams is by means of selective oxidation of CO to CO2 at low temperature (100xc2x0 C.-200xc2x0 C.). In this context it is important that the consumption of hydrogen by non-selective oxidation to water is minimised.
The power of ruthenium (Ru) to catalyse the oxidation of CO is, for example, known from the ammonia synthesis process. Thus, it is known from U.S. Pat. No. 3,216,782 (Nov. 9, 1965) that 0.5% (m/m) Ru on alumina (Al2O3) is capable of oxidising 0.055-0.6% (V/V) CO in the presence of H2 at between 120xc2x0 C. and 160xc2x0 C. to a level of less than 15 ppm. In this case it is necessary that the quantity of oxygen (O2) added is such that the molar O2/CO ratio is between 1 and 2. The excess oxygen which is not needed for the oxidation of CO reacts with hydrogen to give water. It has not been investigated whether this Ru catalyst is also capable of oxidising CO from a typical reformate gas to a CO level of 15 ppm under the same conditions (temperature, O2/CO ratio).
In the Journal of Catalysis 142 (1993), Academic Press Inc., pages 257-259, S. H. Oh and R. M. Sinkevitch describe 0.5% (m/m) Ru/xcex3-Al2O3 as highly effective in the complete oxidation, at low temperature (100xc2x0 C.), of 900 ppm CO with 800 ppm oxygen (O2) in a gas mixture which also contains 0.85% (V/V) H2, with the remainder being N2. Data on the stability of the Ru catalyst are not given in the article and in addition the behaviour of the catalyst in a realistic reformate gas containing H2, CO2, H2O and CO in much higher concentrations was not investigated.
European Patent EP 0 743 694 A1 (Nov. 20, 1996) refers to an oxidation unit for the selective oxidation of CO in H2-rich gas at a reaction temperature of between 80xc2x0 C. and 100xc2x0 C. A molar ratio of O2/CO of 3 is used. The final CO content is a few ppm. The excess oxygen reacts with hydrogen to give water. The catalyst consists of a 0.2% (m/m)-0.5% (m/m) Ptxe2x80x94Ru alloy on Al2O3. No examples which would show the stability of the catalyst are given.
U.S. Pat. No. 5,674,460 (Oct. 7, 1997) describes a structured reactor for the catalytic removal of CO from H2-rich gas at between 90xc2x0 C. and 230xc2x0 C. Depending on the temperature, the catalyst in this case consists of Pt on xcex3-Al2O3, Pt on zeolite-Y or Ru on xcex3-Al2O2. The invention is explained solely on the basis of 5% (m/m) Pt on xcex3-Al2O3, by means of which the CO content can be reduced to about 40 ppm at a reaction temperature of between 80xc2x0 C. and 130xc2x0 C. No stability data are given in this patent either.
In the Journal of Catalysis 168 (1997), Academic Press, pages 125-127, R. M. Torres Sanchez et al. describe gold on manganese oxide as an alternative catalyst for the oxidation of CO in H2 at low temperatures (approximately 50xc2x0 C.). In particular the price, due to the high gold loading (approximately 4-10% (m/m)), makes the use of this type of, catalyst less interesting. Moreover, this type of catalyst is able to withstand carbon dioxide to only a limited extent.
It is not clear from the above whether the catalysts of the prior art are suitable for the selective oxidation of CO in H2-rich reformate gas mixtures where there is high activity in conjunction with good stability in the temperature range 100xc2x0 C.-200xc2x0 C. and where a low oxygen excess can be used to minimise the hydrogen consumption.
The invention relates to a method for the selective catalytic oxidation of carbon monoxide (CO) comprising catalytically oxidizing carbon monoxide in H2-rich, CO2- and H2O-containing gases in the presence of a noble metal catalyst on an xcex1-Al2O3 carrier with the addition of air as oxidizing agent.
One aim of the present invention is to provide a method for the selective catalytic oxidation of CO from H2-rich, CO2- and H2O-containing (reformate) gas mixtures, making use of as small as possible an amount of oxygen and at relatively low temperature. A further aim of the present invention is to provide a catalyst which has high chemical and thermal stability and can be produced in a cost-effective manner by means of a simple method of preparation from commercially available starting materials and a low noble metal loading.
The use of commercially available xcex1-Al2O3 as carrier material in the preparation of 0.5% (m/m) Ru on Al2O3 led, surprisingly, to a catalyst which in the temperature range about 120xc2x0 C. about 160xc2x0 C. combines high activity ( greater than 99% conversion of CO) with high stability (a CO conversion of at least 97% for a period of at least 50 hours) in the oxidation of CO with a relatively small excess of oxygen in dilute reformate gas. These results were found to be appreciably better than the results which were obtained with a commercially available 0.5% (m/m) ruthenium catalyst with xcex3-Al2O3 as the carrier (specific surface area  greater than 100 m2/g), which is representative of the catalysts used in the abovementioned studies and reflects the prior art.
It has also been found that the addition of Pt and the lowering of the total noble metal loading resulted in a catalyst which showed even better stability for the selective oxidation of CO in both dilute and undiluted reformate gas (a CO conversion of at least 99% for a period of at least 50 hours).
It has furthermore been found that in particular the nature and the specific surface area of the Al2O3 carrier used are the factors determining the exceptional performance of the Ru and Ruxe2x80x94Pt catalysts according to the present invention. Preferably, alumina is used in the form of xcex1-Al2O3. A highly active and stable catalyst i formed when the specific surface area of the xcex1-Al2O3 is in the range from about 3 m2/gram to 25 m2/gram.
The catalysts in the present invention can be prepared in a simple manner via a standard impregnation method from commercially available starting materials. Compared with the current state of the art, the method according to the present invention has the following advantages:
complete oxidation of CO to CO2 in the temperature range about 120xc2x0 C. about 160xc2x0 C. with only a small excess of oxygen (O2/CO=1) compared with the stoichiometrically required quantity of oxygen (O2/CO=0.5),
minimal hydrogen consumption as a result of low oxygen excess (O2/CO 1),
stable action at 130xc2x0 C. in simulated reformate gas (0.5% (V/V) CO, 0.5% (V/V) O2, 74% (V/V) H2, 19% (V/V) CO2 and 6% (V/V) H2O) for a period of at least 50 hours (residual quantity of CO  less than 50 ppm),
low noble metal loading of less than about 0.5% (m/m).
xcex1-Al2O3 is a commercial product that is used, inter alia, in the electronics industry in the production of thick and thin substrate layers by tape casting. Another application is the production of industrial ceramics.
The use of this xcex1-Al2O3 as carrier for a selective oxidation catalyst for CO in H2-rich gas mixtures has not been described before.